1. Field of the Invention
The invention relates to an audio power amplifier, and in particular, to an audio compensation method implemented in a class AB power amplifier for distortion compensation.
2. Description of the Related Art
FIG. 1a shows a conventional audio device with a class AB power amplifier. In the audio device 100, a class AB power amplifier 110 receives a reference voltage Vref and an input signal VIN to output an output signal VOUT. A capacitor 102 is coupled to the output signal VOUT for playing the sound. A capacitor 102 is deposited between the class AB power amplifier 110 and the speaker 120, for separating DC bias voltages between the class AB power amplifier 110 and the speaker 120.
As known, the class AB power amplifier 110 has poor linearity characteristics. In practical implementation, the open-loop gain of the class AB power amplifier 110 varies significantly and is a function of the output signal VOUT. Consequently, it introduces severe distortion to the output playback signal.
FIG. 1b shows the symptom of distortion according to FIG. 1a. The waveform of the real output SouT significantly differs from the ideal output waveform especially near to the mid-rail, which is a so-called cross-over distortion. Since linearity is an important feature for audio applications, a class AB power amplifier 110 of poor linearity degrades audio quality. Thus, it is desirable to develop a class AB power amplifier with better linearity for improved audio quality.